A room-temperature approach to boron nitride hollow spheres

Boron nitride hollow spheres were synthesized by the reaction of BBr₃ and NaNH₂ at room temperature; X-ray powder diffraction pattern could be indexed as hexagonal BN with the lattice constants of a=2.482 and c=6.701 Å; high-resolution transmission electron microscopy image showed the hollow spheres consisted of BN nanoparticles, with diameter between 80 and 300 nm; a possible formation mechanism of BN hollow spheres was discussed.     

A self-generated template route to hollow carbon nanospheres in a short time

A self-generated template route was reported to prepare hollow carbon nanospheres. The hollow spheres were obtained through the direct pyrolysis of ferrocene for 1 h. The external diameter of the hollow carbon nanospheres was 50-150 nm and the thickness of the wall was about 15 nm. A possible formation mechanism of the hollow carbon nanospheres was discussed.     

Large-scale synthesis of In2O3 nanowires

In₂O₃ nanowires have been successfully fabricated on a large scale from indium particles by thermal evaporation at 1030 °C. The as-synthesized products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM images show that these nanowires are uniform with diameters of about 60–120 nm and lengths of about 15–25 μm. XRD and selected-area electron diffraction analysis together indicate that these In₂O₃ nanowires crystallize in a cubic structure of the bixbyite Mn₂O₃ (I) type (also called the C-type rare-earth oxide structure). The growth mechanism of these nanowires is also discussed. Received: 29 June 2001 / Accepted: 28 September 2001 / Published online: 20 December 2001     

From nitride to carbide: control of zirconium-based hard materials film growth and their characterization

High-quality thin films of ZrC_yN_1-y and the novel tribological material Zr_0.8Al_0.2C_yN_1-y have been grown by pulsed reactive crossed-beam laser ablation using Zr and Zr–Al ablation targets, respectively, and a pulsed gas. The gas mixture provided the carbon and nitrogen for the solid-solution films. Control of the stoichiometry (i.e. y) was determined by the relative partial pressures of the nitrogen- and carbon-containing gases. It was found that optimal control of the film chemistry was achieved by using the least thermally reactive gases. In this manner, it was possible to activate the gas species exclusively by collisions in the gas phase with the ablation-plume particles, thereby decoupling the chemistry from surface processes. The films were characterized for their chemical, crystallographic, optical, and tribological properties. All the films had very low impurity levels and a cubic rock salt crystal structure over the entire investigated temperature range between 100 and 600 °C. Exceedingly high quality epitaxial films could be grown on MgO (001) at 600 °C. Films grown on stainless steel were polycrystalline. The hardness of the films showed a maximum for both sets for stoichiometries predicted by a recent theoretical model for hardness based on band-structure calculations. In addition, all the films had an exceptionally low coefficient of friction versus steel. Received: 22 August 2001 / Accepted: 3 March 2002 / Published online: 19 July 2002     

Long silicon nitride nanowires synthesized in a simple route

Long silicon nitride (Si₃N₄) nanowires with high purity were synthesized by heating mixtures of SiO₂ powders and short carbon fibers at 1430°C for 2 h in a flowing N₂ atmosphere. The nanowires had the length of 1–2 millimeters and the diameters of 70–300 nm, and were mainly composed of ⍺-Si₃N₄, growing along the [001] direction. The vapor–solid (VS) mechanism was employed to interpret the nanowires growth.     

Effect of laser fluence on the deposition and hardnessof boron carbide thin films

We deposited amorphous thin films of boron carbide by pulsed laser deposition using a B₄C target at room temperature. As the laser fluence increased from 1 to 3 J/cm², the number of 0.25–5 μm particulates embedded in the films decreased, and the B/C atomic ratio of the films increased from 1.8 to 3.2. The arrival of melt droplets, atoms, and small molecular species depending on laser fluence appeared to be involved in the film formation. In addition, with increasing fluence the nanoindentation hardness of the films increased from 14 to 32 GPa. We believe that the dominant factor in the observed increase in the films’ hardness is the arrival of highly energetic ions and atoms that results in the formation of denser films. Received: 23 March 2001 / Accepted: 1 July 2001 / Published online: 2 October 2001     

Theoretical study on structure of boron nitride fullerenes

We propose the parameters of the Stillinger-Weber potential for hexagonal boron nitride (BN) structures. For the reliability of these parameters, the structural property of BN fullerenes is investigated. The stability of BN fullerenes increases with increasing the number of atoms, due to the reduction of the curvature effect of BN fullerenes. The structures of the relative stable fullerenes are B₁₆N₁₆, B₁₈N₁₈, B₂₂N₂₂, B₂₅N₂₅, and B₂₈N₂₈.     

Electrochemical synthesis of polypyrrole/carbon nanotube nanoscale composites using well-aligned carbon nanotube arrays

Polypyrrole/carbon nanotube nanoscale composites were successfully fabricated by electrochemical deposition of polypyrrole over each of the carbon nanotubes in well-aligned large arrays. The thickness of the polypyrrole coating can be easily controlled by the value of the film-formation charge. For both thin (low film-formation charge) and thick (high film-formation charge) films, the polypyrrole coating on the surface of each nanotube is very uniform throughout the entire length, as observed by transmission electron microscopy. Received: 2 May 2001 / Accepted: 4 May 2001 / Published online: 20 June 2001     

Combinatorial synthesis of Li-doped NiO thin films and their transparent conducting properties

Transparent and conductive oxide (TCO) thin films of Li-doped NiO were prepared by combinatorial pulsed laser deposition technique. Composition spread approach was applied to optimize the Li concentration for achieving high conductivity and transparency in the film. Conductivity of the composition spread film was found to increase almost linearly from nearly insulating to as high as 1.41 Ω⁻¹ cm⁻¹ with increasing Li content along a 9 mm long film. Optical transparency was found to decrease in an equal proportion across the 40 nm thick film with a slight shift in the absorption edge.     

First-principles study of transport of V doped boron nitride nanotube

We perform first-principles calculations of spin-dependent quantum transport in V doped boron nitride nanotube: the junction of pristine (6,0) boron nitride nanotube in contact with V doped (6,0) boron nitride nanotube electrodes. Large tunnel magnetoresistance and perfect spin filtration effect are obtained. The zero bias tunnel magnetoresistance is found to be several thousand percent, it reduces monotonically to zero with a voltage scale of about 0.65 V, and eventually goes to negative values after the bias of 0.65 V. The ratio of spin injection is above 95% till the bias of 0.85 V and is even as large as 99% for the bias from 0.25 eV to 0.55 eV when the magnetic configurations of two electrodes are parallel. The understanding of the spin-dependent nonequilibrium transport is presented by investigating microscopic details of the transmission coefficients.     

Structural and electrical properties of tantalum nitride thin films fabricated by using reactive radio-frequency magnetron sputtering

TaN thin film is an attractive interlayer as well as a diffusion barrier layer in [FeN/TaN]_n multilayers for the application as potential write-head materials in high-density magnetic recording. We synthesized two series of TaN films on glass and Si substrates by using reactive radio-frequency sputtering under 5-mtorr Ar/N₂ processing pressure with varied N₂ partial pressure, and carried out systematic characterization analyses of the films. We observed clear changes of phases in the films from metallic bcc Ta to a mixture of bcc Ta(N) and hexagonal Ta₂N, then sequentially to fcc TaN and a mixture of TaN with N-rich phases when the N₂ partial pressure increased from 0.0% to 30%. The changes were associated with changes in the grain shapes as well as in the preferred crystalline orientation of the films from bcc Ta [100] to [110], then to random and finally to fcc TaN [111], correspondingly. They were also associated with a change in film resistivity from metallic to semiconductor-like behavior in the range of 77–295 K. The films showed a typical polycrystalline textured structure with small, crystallized domains and irregular grain shapes. Clear preferred (111) stacks parallel to the substrate surface with embedded amorphous regions were observed in the film. TaN film with [111]-preferred orientation and a resistivity of 6.0 mΩ cm was obtained at 25% N₂ partial pressure, which may be suitable for the interlayer in [FeN/TaN]_n multilayers. Received: 6 December 1999 / Accepted: 24 July 2000 / Published online: 9 November 2000     

Deposition of large-area, high-quality cubic boron nitride films by ECR-enhanced microwave-plasma CVD

Large-area, 1-μm-thick cubic boron nitride (cBN) films were deposited on (001) silicon substrates by electron-cyclotron-resonance-enhanced microwave-plasma chemical vapor deposition (ECR-MP CVD) in a mixture of He-Ar-N₂-BF₃-H₂ gases. With the assistance of fluorine chemistry in the gas phase and substrate reactions, the phase purity of the sp³-configuration was improved to over 85% at a reduced substrate bias voltage of -40 V. The grown films show clear Raman transversal optical (TO) and longitudinal optical (LO) phonon vibration modes, characteristic of cBN. Such Raman spectral characteristics are the first ever observed in cBN films prepared under ECR-MP CVD conditions. Received: 3 May 2002 / Accepted: 7 May 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +852-2788/7830, E-mail: apwjzh@cityu.edu.hk     

Catalytic synthesis of single-crystalline gallium nitride nanobelts

Single-crystalline gallium nitride nanobelts have been synthesized through the reaction of gallium vapor with flowing ammonia using nickel as a catalyst. The as-synthesized products were characterized using X-ray powder diffraction (XRD), scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and selected-area electron diffraction (SAED). XRD and SAED results revealed that the products are pure, single-crystalline GaN with hexagonal structure. The widths and thickness of the nanobelts ranged from 80 to 200 nm, and 10 to 30 nm, respectively. The lengths were up to several tens of micrometers. The nanobelts had smooth surface with no amorphous sheath, and a sharp-tip end. The growth mechanism of nanobelts was discussed.     

Chemical Synthesis and Characterization of Flaky h-BCN at High Pressure and High Temperature

Hexagonal boron carbonitrogen （h-BCN） compound is synthesized from a mixture of boron powder and CNH compound prepared by pyrolysis of melamine （CaH6N6） under high temperature （1400-1500℃） and high pressure （5.0-5.5 GPa）. X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spec- troscopy are used to determine the chemical composition and bonds of the product. The results show that the product has composition of B0.18C0.64N0.16 （near BC4N） and atomic-level hybrid. X-ray diffraction analysis indicates that the powder has a hexagonal network structure. Scanning and transmission electron microscopy results suggest that h-BCN compound morphology is mainly flaky in width about 1 μm and thickness 200nm.     

Simulation of KrF laser ablation of Al2O3-TiC

Previous work by the authors on micromachining of Al₂O₃-TiC ceramics using excimer laser radiation revealed that a columnar surface topography forms under certain experimental conditions. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations show that the columns develop from small globules of TiC, which appear at the surface of the material during the first laser pulses. To understand the mechanism of formation of these globules, a 2D finite element ablation model was developed and used to simulate the time evolution of the temperature field and of the surface topography when a sample of Al₂O₃-TiC composite is treated with KrF laser radiation. Application of the model showed that the surface temperature of TiC rises much faster than that of Al₂O₃, but since TiC has a very high boiling temperature, its vaporization is significant only for a short time. By contrast, the surface temperature of Al₂O₃ rises above its boiling temperature for a much longer period, leading to a greater ablation depth than TiC. As a result, a small TiC globule stands above the Al₂O₃ surface. The results of the model are compared with experimental measurements performed by AFM. After three pulses, the height of the globules predicted by the model is about 340 nm, in good agreement with the height measured experimentally, about 400 nm.     

Effects of Raw Material Content on Efficiency of TiN Synthesized by Reactive Ball Milling Ti and Urea

Ti and urea mixed according to the molar ratios of 2：1, 3：1 and 4：1 axe milled under the same condition. The structures of the as-synthesized powders are analyzed by an x-ray diffractometer （XRD）. The decomposed temperature of the urea and the products decomposed are characterized by differential scanning calorimetry （DSC） and thermogravimetry analysis-Fourier transform infrared （TG-FTIR） spectrometry. The results show that the reaction progress is a diffusion reaction. The efficiency of TiN synthesized by reactive ball milling can be increased by increasing the content of Ti. The reactive ball milling time decreases from more than 90 h to 40 h correspond- ing to the content ratio between Ti and urea increasing from 2：1 to 4：1. Ammonia gas （NH3） and cyanic acid （HNCO）, the decomposed products of urea, react with the refined Ti to form TiN. The grain refinement of Ti has a significant effect on the efficiency of reactive ball milling.     

A reduction–nitridation route to boron nitride nanotubes

Multiwalled boron nitride nanotubes were synthesized through a simple reduction–nitridation route, in which boron trifluoride etherate ((C₂H₅)₂OBF₃) and sodium azide (NaN₃) were used as reactants in the presence of Fe-Ni powder at 600 °C for 12 h. The obtained BN nanotubes have an average outer diameter of 60 nm, an average inner diameter of 30 nm, and an average length up to 300 nm. Some nanobamboo structured BN were found coexisting with the BN nanotubes. The experimental results show that the reaction temperature and Fe-Ni powder play important roles in the formation of BN nanotubes. Finally, a possible formation mechanism is also discussed. PACS 81.07.De; 81.16.Be; 81.16.Rf; 81.16.Hc     

Raman spectrum of single-walled boron nitride nanotube

Using the spectral moments method, the calculations of the Raman spectra of single-walled boron nitride nanotubes (SW-BNNTs) were performed in the framework of the force constants model. Spectra were computed for chiral and achiral nanotubes for different diameters and lengths. The Raman scattering intensities were determined using the bond-polarizability model and a good agreement with group theory analysis was found. We show that the modes in the low frequency region are very sensitive to the nanotube diameter variation, whereas the ones associated to the tangential region are chirality dependent. The number of Raman active modes, their frequencies, and intensities depend on the length of the nanotube.     

Organisation of carbon and boron nitride layers in mixed nanoparticles and nanotubes synthesised by arc discharge

2 bonds attests for the presence of ordered BN domains and of carbon domains; (ii) the elemental profiles show that BN layers and carbon layers are immiscible with a radial organisation into two to five domains; and (iii) the sets of layers at free surfaces – including the inner surfaces of tubes – are always made of carbon. The origin of this chemical organisation, which is most likely obtained during the growth, is discussed. For the hafnium-boride metallic particles coated by C/BN envelopes, a model based on the solidification from the outside to the inside of isolated liquid-like droplets is proposed: the carbon phase solidifies first according to theoretical phase diagrams, and forms the outer shells. For the tubes, a directional eutectic solidification process is shown to account for the observed C/BN/C sequence, in a vapour–liquid–solid scheme, with an hafnium-rich liquid-like particle at the tip of the tube. Received: 26 November 1998 / Accepted: 14 January 1999